Symptoms
When you create a meeting request, you can add Distribution Groups (DGs) or Dynamic Distribution Groups (DDGs) on the To line as required attendees. When a recipient who belongs to either a DG or a DDG accepts the meeting invitation, your original meeting item shows that recipient as an Optional attendee instead of a Required attendee.
Cause
When Outlook receives a meeting acceptance from a member of a DG or a DDG, the email address of the specific sender of the acceptance is compared to the email address for the recipients in the meeting attendee list. Because the attendee list only includes the email address for the DG or DDG, Outlook assumes that the specific member's acceptance is optional and adds the member to the list of optional attendees. This behavior is by design.
Workaround
To work around this issue in Outlook 2016 and later versions, expand the DG or DDG as soon as you add it to the recipient list. If the DG or DDG contains nested DGs or DDGs, expand them as well.Â
